S20A0037abst

S20A0037

We propose for 2 nights of Keck/MOSFIRE observations to follow-up two z∼4 quiescent galaxies in COSMOS as part of our on-going campaign to build a sample of distant quenched/quenching galaxies. From our previous pilot runs (S17B-106 and S18B-040),(1) we have confirmed two z = 4.010 and z=3.775 galaxies, both of which break the record of the highest redshift quiescent galaxy, and (2) we have measured a stellar velocity dispersion of the z=4.010 galaxy (268±59 km s⁻¹), allowing us to perform dynamical analyses for the very first time at z=4. Interestingly, the velocity dispersion is similar to those of massive galaxies today, suggesting that the velocity dispersion does not significantly evolve over the last 12 Gyr. Building upon two papers from our previous runs (Tanaka+, Valentino+), we now propose to observe two similarly bright and distant quiescent galaxy candidates, essentially completing the survey of such massive objects in COSMOS and finally building a meaningful sample. We aim (1) to confirm their redshifts and possibly measure their stellar velocity dispersion, (2) to constrain their star formation histories (quenching timescale), ages, and masses, (3) directly test state-of-the-art modeling and simulations by comparing number densities and mass assembly histories.

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